Small lot size lithography bays

ABSTRACT

In a first aspect, a small lot size lithography bay is provided. The small lot size lithography bay includes (1) a plurality of lithography tools; and (2) a small lot size transport system adapted to transport small lot size substrate carriers to the lithography tools. Each small lot size substrate carrier is adapted to hold fewer than 13 substrates. Numerous other aspects are provided.

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/578,792, filed Jun. 10, 2004. This application is also a continuation-in-part of and claims priority to U.S. patent application Ser. No. 10/764,620, filed Jan. 26, 2004, now U.S. Pat. No. 7,221,993 which claims priority to U.S. Provisional Application Ser. No. 60/443,001, filed Jan. 27, 2003. The content of each of the above applications is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor device fabrication systems, and more particularly to small lot size lithography bays.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is related to the following commonly-assigned, co-pending U.S. patent applications, each of which is hereby incorporated by reference herein in its entirety:

-   U.S. patent application Ser. No. 10/650,310, filed Aug. 28, 2003 and     titled “System For Transporting Substrate Carriers”; -   U.S. patent application Ser. No. 10/650,312, filed Aug. 28, 2003 and     titled “Method and Apparatus for Using Substrate Carrier Movement to     Actuate Substrate Carrier Door Opening/Closing”; -   U.S. patent application Ser. No. 10/650,481, filed Aug. 28, 2003 and     titled “Method and Apparatus for Unloading Substrate Carriers from     Substrate Carrier Transport Systems”; -   U.S. patent application Ser. No. 10/650,479, filed Aug. 28, 2003 and     titled “Method and Apparatus for Supplying Substrates to a     Processing Tool”; -   U.S. patent application Ser. No. 60/407,452, filed Aug. 31, 2002 and     titled “End Effector Having Mechanism For Reorienting A Wafer     Carrier Between Vertical And Horizontal Orientations”; -   U.S. patent application Ser. No. 60/407,337, filed Aug. 31, 2002,     and titled “Wafer Loading Station with Docking Grippers at Docking     Stations”; -   U.S. patent application Ser. No. 10/650,311, filed Aug. 28, 2003 and     titled “Substrate Carrier Door having Door Latching and Substrate     Clamping Mechanism”; -   U.S. patent application Ser. No. 10/650,480, filed Aug. 28, 2003 and     titled “Substrate Carrier Handler That Unloads Substrate Carriers     Directly From a Moving Conveyor”; -   U.S. patent application No. 10/764,982, filed Jan. 26, 2004 and     titled “Methods and Apparatus for Transporting Substrate Carriers”; -   U.S. patent application No. 10/764,820, filed Jan. 26, 2004, and     titled “Overhead Transfer Flange and Support for Suspending     Substrate Carrier”; -   U.S. Provisional Patent Application Ser. No. 60/443,115, filed Jan.     27, 2003, and titled “Apparatus and Method for Storing and Loading     Wafer Carriers”; -   U.S. patent application Ser. No. 10/987,956, filed Nov. 12, 2004,     and titled “Calibration of High Speed Loader to Substrate Transport     System”; and -   U.S. Provisional patent application Ser. No. 60/520,035, filed Nov.     13, 2003, and titled “Apparatus and Method for Transporting     Substrate Carriers Between Conveyors”.

BACKGROUND OF THE INVENTION

Manufacturing of semiconductor devices typically involves performing a sequence of procedures with respect to a substrate such as a silicon substrate, a glass plate, etc. These steps may include polishing, deposition, etching, photolithography, heat treatment, and so forth. Usually a number of different processing steps may be performed in a single processing system or “tool” which includes a plurality of processing chambers. However, it is generally the case that other processes are required to be performed at other processing locations within a fabrication facility, and it is accordingly necessary that substrates be transported within the fabrication facility from one processing location to another. Depending on the type of semiconductor device to be manufactured, there may be a relatively large number of processing steps required, to be performed at many different processing locations within the fabrication facility.

It is conventional to transport substrates from one processing location to another within substrate carriers such as sealed pods, cassettes, containers and so forth. It is also conventional to employ automated substrate carrier transport devices, such as automatic guided vehicles, overhead transport systems, substrate carrier handling robots, etc., to move substrate carriers from location to location within the fabrication facility or to transfer substrate carriers from or to a substrate carrier transport device.

For an individual substrate, the total fabrication process, from formation or receipt of the virgin substrate to cutting of semiconductor devices from the finished substrate, may require an elapsed time that is measured in weeks or months. In a typical fabrication facility, a large number of substrates may accordingly be present at any given time as “work in progress” (WIP). The substrates present in the fabrication facility as WIP may represent a very large investment of working capital, which tends to increase the per substrate manufacturing cost.

When a fabrication facility is fully operational, reducing WIP decreases capital and manufacturing costs. WIP reduction may be achieved, for example, by reducing the average total elapsed time for processing each substrate within the fabrication facility.

Previously incorporated U.S. patent application Ser. No. 10/650,310, filed Aug. 28, 2003, titled “System for Transporting Semiconductor Substrate Carriers”, discloses a substrate carrier transport system that includes a conveyor for substrate carriers that is intended to be constantly in motion during operation of the fabrication facility which it serves. The constantly moving conveyor is intended to facilitate transportation of substrates within the fabrication facility so as to reduce the total “dwell” or “cycle” time of each substrate in the fabrication facility. WIP reduction thereby may be achieved as less WIP is needed to produce the same factory output.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method is provided that includes the steps of (1) storing substrates in a plurality of small lot size substrate carriers, each adapted to hold less than 13 substrates; and (2) transporting at least one of the small lot size substrate carriers within a lithography bay of a semiconductor device fabrication facility.

In a second aspect of the invention, a small lot size lithography bay is provided. The small lot size lithography bay includes (1) a plurality of lithography tools; and (2) a small lot size transport system adapted to transport small lot size substrate carriers to the lithography tools. Each small lot size substrate carrier is adapted to hold fewer than 13 substrates. Numerous other aspects are provided.

Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an exemplary graph of semiconductor device fabrication facility (FAB) cycle time or WIP versus facility output for large and small lot size substrate carriers.

FIG. 2 is a schematic diagram of an exemplary small lot size (SLS) semiconductor device manufacturing facility provided in accordance with the present invention.

FIG. 3 is a top plan view of an exemplary small lot size lithography bay provided in accordance with the present invention.

FIG. 4 is a flowchart of an exemplary method for operating the small lot size lithography bay of FIG. 3.

DETAILED DESCRIPTION

The present invention relates to reducing cycle time in a semiconductor device fabrication facility, such as in a lithography portion (bay) of a semiconductor device fabrication facility. Cycle time is reduced by reducing lot size and/or by employing a high speed transport system to transport small lot size substrate carriers between processing, metrology and/or inspection tools of the lithography bay.

As used herein, a “small lot” size substrate carrier refers to a substrate carrier that is adapted to hold significantly fewer substrates than a conventional “large lot” size substrate carrier which typically holds 13 or 25 substrates. As an example, in one embodiment, a small lot size substrate carrier is adapted to hold 5 or less substrates. Other small lot size substrate carriers may be employed (e.g., small lot size carriers that hold 1, 2, 3, 4, 5, 6, 7 or more substrates, but significantly less than that of a large lot size substrate carrier). For example, in one embodiment, each small lot size substrate carrier may hold too few substrates for human transport of substrates carriers to be viable within a semiconductor device manufacturing facility.

Cycle Time Reduction

In a conventional semiconductor device fabrication facility, substrates are typically transported in substrate carriers that hold 25 substrates per carrier. In such cases, the fabrication facility is said to employ a lot size of 25 substrates.

The cycle time (CT) to process a substrate lot can be determined using the following formula: CT=lot processing time+transit time+tool wait time Lot processing time (LPT) is the time required to process each substrate in a lot (e.g., 25 substrates for a standard lot size). Lot processing time includes, for example, the time required to transfer each substrate from its substrate carrier to a processing chamber of a processing tool, process the substrate and return the substrate to its substrate carrier. Transit time (TT) refers to the time required to transfer a lot (e.g., a substrate carrier) from one processing tool to another processing tool (e.g., the time between when a substrate carrier is closed at a first tool and opened at a second tool).

Tool wait time (TWT) refers to the time that a lot must wait at a processing tool before substrates in the lot are processed at the tool. Assuming each lot is processed immediately after arriving at each processing tool, the tool wait time is zero, and the minimum cycle time (CTMIN) becomes:, CTMIN=lot processing time+transit time From the above formula, it is clear that for a fixed transit time, cycle time increases with lot size (because each substrate within a lot generally must wait for all other substrates in the lot to be processed before the lot may be transferred to a new processing tool). Accordingly, by employing small lot sizes (e.g., substrate carriers that hold fewer substrates), cycle time can be reduced as long as transit time is not significantly increased. By using a lot size of 1 substrate (e.g., a substrate carrier that holds only one substrate carrier) without increasing transit time, cycle time may be minimized because no substrate must wait for other substrates to be processed before being transported to a new processing tool for its next processing step.

As an example, assume that for 130 nanometer logic, process time is 2 days and transit time is 2 days. Further assume for this example that, for a 25 substrate lot size, the time that each substrate must wait for other substrates in its lot to be processed is 10 days. Minimum cycle time for the 25 substrate lot (CTMIN25) becomes:

$\begin{matrix} {{{CT}\;{MIN}\; 25} = {{{process}\mspace{14mu}{time}} + {{lot}\mspace{14mu}{wait}\mspace{14mu}{time}} + {{transit}\mspace{14mu}{time}}}} \\ {= {{2\mspace{14mu}{days}} + {10\mspace{14mu}{days}} + {2\mspace{14mu}{days}}}} \\ {= {14\mspace{14mu}{days}}} \end{matrix}$ However, assuming a single substrate lot size is employed, and that transit time remains fixed, the minimum cycle time for the 1 substrate lot (CTMIN1) becomes:

$\begin{matrix} {{{CT}\;{MIN}\; 1} = {{{process}\mspace{14mu}{time}} + {{lot}\mspace{14mu}{wait}\mspace{14mu}{time}} + {{transit}\mspace{14mu}{time}}}} \\ {= {{2\mspace{14mu}{days}} + {0\mspace{14mu}{days}} + {2\mspace{14mu}{days}}}} \\ {= {4\mspace{14mu}{days}}} \end{matrix}$ Accordingly, by employing a single substrate lot size, minimum cycle time is reduced by 73%.

From a cycle time perspective, the use of single substrate carriers may appear to be preferred. However, other issues such as providing sufficient storage at each processing tool, scheduling lots, process control strategies or the like, may make the use of lots sizes greater than 1 substrate desirable. For example, it may be desirable to use small lot sizes having 2, 3, 4, 5, 6 or more substrates. Note that the use of such small lot sizes still provides a significant reduction in cycle time compared to conventional large lot sizes.

Cycle time and its affects on fabrication facilities are described below with reference to FIGS. 1 and 2. In particular, FIGS. 1 and 2 describe managing work in progress (WIP) and cycle time through the use of hot lots. Manipulating cycle time within a small lot size lithography bay is described below with reference to FIGS. 3 and 4.

Cycle Time Versus WIP in Fabrication Facilities

As stated, for a fully operational fabrication facility, reducing WIP decreases capital and manufacturing costs. However, reduced WIP also may place a semiconductor device manufacturing facility as risk. For instance, when a processing tool within a fabrication line becomes non-operational (e.g., “goes down” due to equipment failure, for periodic maintenance or cleaning, etc.), WIP may provide sufficient substrate buffering to allow continued factory output until the non-operational tool becomes operational (e.g., is brought “back on line”). Insufficient WIP, on the other hand, may cause the manufacturing line to sit idle.

In general, when average cycle time per substrate is reduced within a semiconductor device manufacturing facility, a corresponding reduction (e.g., a similar, proportional or otherwise-related reduction) in work in progress is realized as, on average, more substrates may be moved through the facility. In accordance with at least one embodiment of the present invention, average cycle time per substrate may be maintained approximately constant by increasing the cycle time of low priority substrates while decreasing the cycle time of high priority substrates. (Note that low and high priority substrates may be distributed over a range of priority values, such as from priority 1 to priority 100 or over some other suitable priority range). In this manner, the relationship between the cycle time of high priority substrates and work in progress may be decoupled (e.g., reduced or eliminated) such that a decrease in high priority substrate cycle time does not produce a corresponding decrease in work in progress within a semiconductor device fabrication facility employing the present invention.

To reduce average cycle time in accordance with the present invention, “small lot” size substrate carriers are employed with a high speed substrate carrier transport system. The high speed substrate carrier transport system may transport substrate carriers within a semiconductor device fabrication facility at a significantly higher speed than conventional transport systems (as described below). Accordingly, any given substrate carrier may be routed through the facility faster.

Use of Small Lot Sizes During Semiconductor Device Manufacturing

FIG. 1 is an exemplary graph of semiconductor device fabrication facility (FAB) cycle time or WIP versus facility output for large and small lot size substrate carriers. With reference to FIG. 1, curve 100 illustrates cycle time or WIP versus facility output for a typical semiconductor device fabrication facility configured to transport large lot size substrate carriers. Curve 102 illustrates cycle time or WIP versus facility output for a semiconductor device fabrication facility configured to transport small lot size substrate carriers in accordance with the present invention.

Curve 100 illustrates how facility output increases with WIP. As WIP continues to increase, facility output also increases. Eventually large WIP increases are required for relatively small output gains. Accordingly, to provide facility output predictability, large lot size facilities typically operate near the knee of curve 100 (as represented by reference numeral 104). At such a location, facility output is near its maximum and small increases or decreases in cycle time or WIP have little effect on facility output.

Curve 102 illustrates how use of small lot size substrate carriers with a high speed substrate carrier transport system (described below) can affect WIP and/or facility output. For example, as shown in FIG. 1, curve 102 is shifted to the right and down relative to curve 100. Such a shift indicates that the same level of facility output can be maintained with less WIP (as indicated by reference numeral 106). Alternatively, if the same level of WIP is desired within the small lot size facility as is present in the large lot size facility governed by curve 100, output within the small lot size facility may be increased (as in indicated by reference numeral 108). In at least one embodiment of the invention, factory output is increased through a slight reduction in WIP, as shown, for example, by reference numeral 110. Other operating points along the curve 102 also may be employed.

Selection of the “optimal” operating point on the curve 102 depends on numerous factors. For example, some products have long product life cycles and relatively stable prices (e.g., devices for embedded applications such as devices used within industrial equipment or other long-product-life applications). Devices used in these products can be fabricated and stored in inventory for sale at later dates with little financial risk. Substrates for such devices may be classified as low priority and used to fill capacity of the fabrication facility. As inventories become depleted, the priority for these substrates may be increased as required to meet committed business arrangements (e.g., lead time, stocking levels, etc.).

Substrates for devices used in mature, commodity products, such as DRAM products that have been on the market for 3-4 years, also are candidates for low priority substrates. Yields for these devices are generally high, and the commodity nature of the product employing the devices ensures that a market exists. Accordingly, inventory obsolescence is unlikely, but profit potential is relatively low. However, such substrates may provide capacity fill in to ensure that equipment is continually utilized to build revenue generating product.

New products characteristically provide high gross margins. Substrates for devices used in such products may be given a high priority. In this manner, the fabrication facility may increase profit per substrate by biasing production to higher gross margin products. New products often experience rapid selling erosion due to competition and marketing strategies that push lower pricing to expand market penetration. Placing higher priority on these products and selectively reducing the manufacturing cycle time may increase the product volume that can be sold at higher gross margin. Substrates for devices used in products with a short-life cycle also may be given a high priority. Short life cycle products may include, for example, customized devices for consumer electronics such as specialized memory devices for digital cameras, video game controllers, cell phone components, etc. More risk exists in creating an inventory of these type of devices as prices may drop rapidly as they approach obsolescence.

Special order devices or devices under development, undergoing changes or undergoing special quality tests, are often handled specially in fabrication facilities; and substrates used for the fabrication of such devices are often referred to as “hot lots” or “super hot lots”. Hot lots are often prioritized to be moved to the front of a queue for each process step. Super hot lots may be given higher priority and equipment may be reserved, or kept idle, waiting for the super hot lot substrates to arrive. In a conventional fabrication facility that employs 13 or 25 substrates per lot, the use of hot lots and super hot lots may cause significant disruptions in process flow. However, as described further below, in a small lot size fabrication facility provided in accordance with the present invention, special order and other similar devices may be interlaced into the existing production flow with little loss of tool utilization. Substrates used for the fabrication of such devices may be given the highest priority within the fabrication facility.

Exemplary Small Lot Size Semiconductor Device Manufacturing Facility

FIG. 2 is a schematic diagram of an exemplary small lot size (SLS) semiconductor device manufacturing facility 200 provided in accordance with the present invention. With reference to FIG. 2, the SLS facility 200 includes a high speed substrate carrier transport system 202 adapted to deliver small lot size substrate carriers to a plurality of processing tools 204. Local storage or buffering 206 is provided at or near each processing tool 204 for local storage of WIP. Additional, volume storage or buffering 208 also may be provided (e.g., volume stocking for accommodating peaks in WIP that develop during manufacturing and for longer term storage of WIP).

Carrier opening devices 210 are provided at each processing tool 204 for opening small lot size substrate carriers so that substrates contained therein may be extracted therefrom, processed and/or returned thereto. Mechanisms (not separately shown) are provided for transferring small lot size substrate carriers from the high speed transport system 202 to the carrier opening devices 210 of each processing tool 204 as described further below.

An automation communications and/or software system 212 is provided for controlling operation of the fabrication facility 200, and may include, for example, a manufacturing execution system (MES), a Material Control System (MCS), a Scheduler or the like. A separate delivery system controller 214 for controlling delivery of small lot size substrate carriers to the processing tools 204 is shown in FIG. 2. It will be appreciated that the delivery system controller 214 may be part of the automation communications and/or software system 212; and/or that a separate MES, MCS or Scheduler may be employed.

Most processing tools, even in a large lot size fabrication facility, process substrates in small batches (e.g., by processing one or two substrates per processing chamber at a time). However, some processing tools, such as furnaces or wet processing tools, process substrates in large batch sizes (e.g., from about 25 to 200 substrates per batch). Accordingly, the small lot size fabrication facility 200 may be adapted to accommodate the delivery, storage and operating requirements of large batch size processing tools. One such exemplary large batch size processing tool is indicated by reference numeral 216 in FIG. 2.

Many large batch size processing tools utilize an equipment front end module (EFEM), not separately shown, to remove substrates directly from a large lot size carrier with a robot blade (e.g., one substrate at a time). Individual substrates then are transferred by the processing tool to a process chamber as required to build the necessary large batch for processing. By providing local storage of small lot size substrate carriers at such a processing tool, a large substrate batch may be built using an equipment front end module of a large batch size processing tool within the small lot size fabrication facility 200.

Other large batch size processing tools pull complete large lot size carriers into an internal buffer, or simultaneously pull multiple substrates from a large lot size substrate carrier into a internal buffer. For such tools, a sorter module 218 that moves substrates from small lot size carriers to large lot size carriers may be employed within the small lot size fabrication facility 200.

As stated, the high speed transport system 202 is adapted to deliver small lot size substrate carriers to the plurality of processing tools 204. Such a system preferably has a move rate capacity of at least 2 times the average rate required for normal production capacity (e.g., to be able to respond to peaks in move rate requirements that occur during normal production). Additionally, the system preferably has the ability to re-direct, re-route or re-move substrates to different processing tools in response to factory anomalies, excursions and/or manufacturing processing changes (e.g., un-scheduled maintenance, priority changes, manufacturing yield problems, etc.).

U.S. patent application Ser. No. 10/650,310, filed Aug. 28, 2003 (the '310 Application), discloses a substrate carrier transport system (e.g., a conveyor) that may be used as the small lot size substrate carrier transport system 202. The transport system of the '310 Application continues to move during substrate carrier unloading and loading operations. A load/unload mechanism may be associated with each processing tool or group of processing tools and operate to load and/or unload substrate carriers from or onto the transport system while the transport system is moving. Each load/unload mechanism may include a load/unload member that is moved during a load or unload operation so as to substantially match the velocity at which the transport system carries substrate carriers. The load and/or unload member is thus adapted to ensure gentle substrate/substrate carrier handling. The transport system of the '310 Application may be operated at significantly higher speeds than conventional large lot size transport systems; and can easily accommodate re-directing, re-routing and/or re-moving of substrates to different processing tools in response to factory anomalies, excursions and/or manufacturing processing changes. Other substrate carrier transport systems may be similarly employed.

One particular embodiment of a suitable high speed transport system is described in U.S. patent application Ser. No. 10/764,982, filed Jan. 26, 2004 (the '982 Application). The '982 Application describes a conveyor system that may include a ribbon of stainless steel or a similar material that forms a closed loop within at least a portion of a semiconductor device manufacturing facility and that transports substrate carriers therein. By orienting the ribbon so that a thick portion of the ribbon resides within a vertical plane and a thin portion of the ribbon resides within a horizontal plane, the ribbon is flexible in the horizontal plane and rigid in the vertical plane. Such a configuration allows the inventive conveyor to be constructed and implemented inexpensively. For example, the ribbon requires little material to construct, is easy to fabricate and, due to its vertical rigidity/strength, can support the weight of numerous substrate carriers without supplemental support structure (such as rollers or other similar mechanisms used in conventional, horizontally-oriented belt-type conveyor systems). Furthermore, the conveyor system is highly customizable because the ribbon may be bent, bowed or otherwise shaped into numerous configurations due to its lateral flexibility.

As shown in FIG. 2, the SLS fabrication facility 200 may include additional high speed transport systems as indicated by reference numerals 202′ and 202″. Few or more than three such transport systems may be employed. Additional high speed transport systems may be employed, for example, to transfer small lot size substrate carriers to other portion of a fabrication facility, such as to equipment that processes substrates stored in large lot size substrate carriers, different physical areas of the fabrication facility (such as an expanded area of the facility), etc. One or more transfer mechanisms 220 may be employed to transfer substrate carriers between the high speed transport systems 202, 202′, and 202″ and/or to provide additional substrate carrier storage. In at least on embodiment of the invention, one or more substrate carrier handlers and a rotary substrate carrier stage may be employed to remove a substrate carrier from a first high speed transport system (e.g., high speed transport system 202 in FIG. 2) and transfer the substrate carrier to a second high speed transport system (e.g., high speed transport system 202′ in FIG. 2) or vice versa. Such a system and method is described in U.S. Provisional Patent Application Ser. No. 60/520,035, filed Nov. 13, 2003.

To avoid depletion of substrate carriers from the high speed transport system 202, each processing tool 204 may include local storage for buffering or storing WIP (as stated). Stand-alone operation of a processing tool without needing the high speed transport system 202 to deliver substrates to the tool thereby may be provided. Because many conventional processing tools accommodate at least two large lot size (e.g., 25) substrate carriers, in at least one embodiment of the invention, local storage at or near each processing tool 204 is adapted to store a similar number of substrates by storing numerous small lot size substrate carriers at or near each processing tool (e.g., about 50 or more small lot size carriers in one embodiment). Other or different numbers of substrate carriers also may be stored at or near each processing tool 204 (e.g., fewer than 50, more than 50, etc.).

U.S. Provisional Application Ser. No. 60/443,115, filed Jan. 27, 2003, discloses a high-speed bay-distributed stocker (HSBDS) having a chassis which is notably longer in a direction of travel of a high speed transport system it serves, as compared to conventional bay-distributed stockers. The HSBDS features a high-speed substrate carrier handler adapted to load substrate carriers onto and unload substrate carriers from a high speed transport system. The HSBDS also includes additional columns of substrate carrier storage shelves for providing additional substrate buffering. Other systems for providing local storage may be employed (e.g., distributed stockers, field stockers, overhead/ceiling mounted tables or shelves, etc).

Volume storage or buffering 208 may provide volume stocking for accommodating peaks in WIP that develop during manufacturing and for longer term storage of WIP. For example, orders placed on hold, non-product substrates, and the like may be placed in volume storage or buffering. Selection of the capacity of such volume storage or buffering is driven by fabrication facility requirements.

Volume storage or buffering 208 may include, for example, small lot size stockers along a high speed transport loop, large lot size stockers on the high speed transport path or along a slower transport path (not shown), another type of high density storage, distributed stockers, field stockers, overhead and/or ceiling mounted tables or shelves, etc. Substrates stored in small lot size substrate carriers may be transferred to large lot size carriers via a sorter, and subsequently placed in a high density storage location.

In at least one embodiment of the invention, work in progress that is low usage (e.g., inactive WIP such as substrates put on engineering hold, test substrates, non-production substrates, substrates built for long term hold, etc.) may be stored in large lot size substrate carriers that have greater storage capacity (as the cost of storing substrates within such high density carriers is generally cheaper than using small lot size carrier storage). For example, low use WIP may be transferred from small lot size carriers into large lot size carriers (e.g., via a sorter) and stored in volume stockers. If the low use WIP is not to be processed within a predetermined time period (e.g., 5 days or some other time period), the WIP may be transferred from small lot size carriers to large lot size carriers (e.g., carriers that store 13 or 25 substrates) and transferred to another location within the fabrication facility 200. For example, volume storage 208′ may include large lot size volume stockers adapted to store low usage WIP at a location away from the main processing area of the fabrication facility 200.

During loading and unloading of substrate carriers at a processing tool 204, communication between the processing tool 204 and the fabrication facility 200 may provide lot identification information, processing parameters, tool operating parameters, processing instructions, or the like. The automation communications and/or software system 212 is designed to handle this and other communications. Preferably such communication is rapid enough so as not to delay substrate processing. For example, a typical requirement for loading of a substrate carrier at a factory interface of a processing tool may be less than about 200 seconds, and in some cases less than 30 seconds.

In at least one embodiment of the invention, the automation communication and/or software system 212 is adapted to perform substrate-level tracking (as opposed to carrier level tracking typically employed in a large lot size environment) and may reference substrates grouped as a lot if necessary. Such a substrate-based approach may increase performance of the fabrication facility 200 and may prevent software restrictions from negating the benefits of small lot size manufacturing.

The carrier opening devices 210 at each processing tool 204 (for opening small lot size substrate carriers so that substrates contained therein may be extracted therefrom, processed and/or returned thereto) preferably employ industry standard interfaces (e.g., SEMI standards) to minimize facility-wide implementation costs. Alternative carrier opening mechanisms may be employed. For example, U.S. patent application Ser. No. 10/650,311, filed Aug. 28, 2003, discloses a door latching mechanism of a substrate carrier that is automatically unlatched by interaction of the latching mechanism with an actuator mechanism at a substrate transfer location (e.g., of a processing tool that may be used, for example, during semiconductor device manufacturing). The same actuator mechanism also may release a substrate clamping mechanism that may be part of the substrate carrier (e.g., and that secures a substrate stored by the substrate carrier during transport). Likewise, U.S. patent application Ser. No. 10/650,312, filed Aug. 28, 2003, describes the use of movement of a substrate carrier toward a port of a substrate transfer location to cause the door of the substrate carrier to open. Movement of the substrate carrier away from the port of the substrate transfer location causes the door of the substrate carrier to close. Other carrier opening methods and apparatus may be employed.

Because of the relatively low move rates employed within large lot size fabrication facilities, human operators may move lots between processing tools when needed (e.g., when an automated transport system fails). For small lot size manufacturing, the required move rate is so large that use of human operators is typically impractical. As a result, redundant systems (other than human operators) may be employed to ensure proper facility operation. Such redundant systems may include, for example, additional control system computers, software databases, automated transport systems and the like (not separately shown).

In at least one embodiment of the invention, the small lot size fabrication facility 200 may include, for example, the ability to install and set up new processing tools while the high speed transport system 202 is operating (e.g., is in motion and/or servicing other tools). U.S. patent application Ser. No. 10/987,956, filed Nov. 12, 2004 , describes methods by which a high speed substrate carrier transfer station (for loading substrate carriers onto and for removing substrate carriers from a high speed substrate carrier transport system) may be aligned and calibrated to a high speed substrate carrier transport system while the transport system is in motion. The substrate carrier hand-off function of the transfer station may then be tested, and the high-speed transfer station placed into service. Other methods/systems also may be employed.

Use of a small lot size fabrication facility such as that described above with reference to FIG. 2 provides numerous advantages over conventional large lot size fabrication facilities. For example, a small lot size fabrication facility may provide greater versatility. As described with reference to FIG. 1, for an equivalent set of processing tools, a small lot size fabrication facility may be operated at (1) equivalent output with reduced cycle time; (2) equivalent cycle time with increase factory output; or (2) at some point therebetween. The facility operating conditions (versus a large lot size baseline) may be tuned based on business needs or other conditions to place priority on either cycle time reduction or increased capacity. Further, through use of the high speeds substrate carrier transport system, special order and other similar devices may be interlaced into the existing production flow with little loss of tool utilization. Hot lots and/or super hot lots typically may be processed within the small lot size fabrication facility with less impact to fabrication facility output rate. Less excess capacity thereby may be required to process hot lots and/or super hot lots.

The foregoing description discloses only exemplary embodiments of the invention. Modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, other configurations for a small lot size fabrication facility may be employed such as different processing tool, storage device and/or transport system layouts and/or types, more or fewer processing tools, storage devices and/or transport systems, etc. In at least one embodiment, a substrate carrier cleaner 222 may be provided for cleaning small lot size substrate carriers. In this manner substrate carriers may be cleaned so as to prevent cross-contamination of incompatible processes. A small lot size semiconductor device manufacturing facility such as that shown in FIG. 2 may form a part or subset of a larger semiconductor device manufacturing facility (e.g., that may include one or more other small lot size semiconductor device manufacturing facilities and/or one or more large lot size semiconductor device manufacturing facilities). For instance, a small lot size semiconductor device manufacturing facility may be employed to reduce the cycle time of all or a portion of an interconnect formation phase of device production. That is, a small lot size semiconductor device manufacturing facility provided in accordance with the invention may be employed to selectively accelerate a portion of device processing time. As another example, in a lithography bay, a small lot size fabrication module may be used to improve the cycle time for metrology, substrate re-works, etc.

The automation communication and/or software system 212, the delivery system controller 214 and/or any other controller may be programmed or otherwise configured to perform numerous WIP management functions. For instance, in at least one embodiment of the invention, such a system and/or controller may be configured to maintain a predetermined work in progress level within the small lot size semiconductor device manufacturing facility 200 by (1) increasing an average cycle time of low priority substrates within the small lot size semiconductor device manufacturing facility 200; and (2) decreasing an average cycle time of high priority substrates within the small lot size semiconductor device manufacturing facility 200 so as to approximately maintain a predetermined work in progress level within the small lot size semiconductor device manufacturing facility 200. In another embodiment of the invention, the system and/or controller may be configured to (1) store small lot size substrate carriers containing low priority substrates within small lot size substrate carrier storage locations of one or more of the processing tools 204; and (2) process high priority substrates available to the one or more of the processing tools 204 ahead of the stored low priority substrates so as to reduce cycle time of high priority substrates without correspondingly reducing work in progress within the small lot size semiconductor device manufacturing facility 200.

In yet another embodiment of the invention, such a system and/or controller may be configured to (1) store small lot size substrate carriers containing low priority substrates and small lot size substrate carriers containing high priority substrates within small lot size substrate carrier storage locations of one or more of the processing tools 204; and (2) prior to processing within the one or more of the processing tools 204, store high priority substrates for a shorter time period on average than low priority substrates so as to reduce cycle time of high priority substrates without correspondingly reducing work in progress within the small lot size semiconductor device manufacturing facility 200.

In a further embodiment of the invention, such a system and/or controller may be configured to process high and low priority substrates within the small lot size semiconductor device manufacturing facility 200 with different cycle times while keeping average cycle time and work in progress at approximately the same level as average cycle time and work in progress of a large lot size semiconductor device manufacturing facility.

In another embodiment of the invention, such a system and/or controller may be configured to process substrates within the small lot size semiconductor device manufacturing facility 200 with a lower average cycle time than a large lot size semiconductor device manufacturing facility while maintaining approximately the same overall output as the large lot size semiconductor device manufacturing facility.

In yet a further embodiment of the invention, such a system and/or controller may be configured to process substrates within the small lot size semiconductor device manufacturing facility 200 with approximately the same average cycle time and work in progress as a large lot size semiconductor device manufacturing facility while increasing output of the small lot size semiconductor device manufacturing facility 200 relative to the large lot size semiconductor device manufacturing facility.

In still a further embodiment of the invention, such a system and/or controller may be configured to (1) identified work in progress that is not to be processed within a predetermined time period; (2) transfer the identified work in progress from small lot size substrate carriers to large lot size substrate carriers; and store the large lot size substrate carriers in volume storage.

Small Lot Size Lithography Bay

Reduced cycle time is particularly advantageous for the lithography portion (bay) of a semiconductor device fabrication facility. FIG. 3 is a top plan view of an exemplary small lot size lithography bay 300 provided in accordance with the present invention. With reference to FIG. 3, the lithography bay 300 includes a wet clean processing tool 302, a dry strip processing tool 304, a plurality of metrology and inspection tools 306-312 and a plurality of patterning tools 314-328. Note that other numbers and/or types of wet clean, dry strip, metrology and inspection and/or patterning tools may be used. For example, separate metrology and inspection tools may be used.

The wet clean processing tool 302 and the dry strip processing tool 304 are “re-work” tools adapted to perform conventional photoresist or other mask removal processes, or any other conventional processes for re-working substrates improperly processed during lithography. For example, the wet clean processing tool 302 may be adapted to apply photoresist stripper or other wet chemical baths to substrates. Likewise, the dry strip processing tool 304 may employ plasma processing (e.g., ashing) to re-work substrates. Other re-work processes and/or tools may be employed within the lithography bay 300, whether in line with the other tools 306-328 (as shown) or otherwise located.

The metrology and inspection tools 306-312 comprise conventional metrology and inspection tools for determining, for example, critical dimensions, overlay accuracy, defect levels and/or defect types, etc., of lithographically processed substrates. In at least one embodiment, the metrology and inspections tools 306-312 may include single substrate, stand alone tools although batch and/or integrated metrology and inspection tools may be used.

The patterning tools 314-328 may include conventional lithography tools that perform one or more of and/or any combination of resist processing, exposure, imprint, etc. For example, a patterning tool may include a stand alone resist processing tool, a stand alone exposure tool, a stand alone imprint tool, an integrated resist processing and exposure tool, an integrated resist processing and imprint tool, or the like. In at least one embodiment, a resist processing tool may be integrated with or otherwise connected to an exposure tool such that (1) a substrate carrier is delivered to the resist processing tool; (2) a substrate or substrates are removed from the substrate carrier and resist (e.g., photoresist) is applied to the substrate or substrates using the resist processing tool; (3) the substrate or substrates are transferred to the exposure tool and exposed; (4) the substrate and/or substrates are returned to the resist processing tool and the applied resist is developed; and (5) the substrate or substrates are returned to the substrate carrier.

The patterning tools 314-328 may be single substrate or batch tools. As stated, other numbers and/or types of wet clean, dry strip, metrology and inspection and patterning tools may be used. In one or more embodiments, any exposure tools that are employed may use visible or ultra-violet light to transfer circuit patterns from a mask to a resist layer. Likewise, direct transfer, extreme ultra-violet light, e-beam or x-ray lithography tools may be employed.

In the embodiment of FIG. 3, a small lot size transport system 330 is employed to transport small lot size substrate carriers between the tools 302-328. The small lot size transport system 330 may be similar to the high speed transport system 202 described previously with reference to FIG. 2, or another suitable transport system may be used. More than one small lot size transport system may be used to transfer carriers between the tools 302-328.

As further shown in FIG. 3, the small lot size lithography bay 300 includes one or more substrate carrier stockers or similar devices 332-336. The stockers 332-336 may be employed to receive large lot size substrate carriers from a conventional large lot size substrate carrier transport system 338 (such as a conventional overhead transport system that transports substrate carriers that hold 25 substrates). In the embodiment shown, the stockers 332-336 are adapted to receive large lot size substrate carriers from the transport system 338, and transfer substrates within each large lot size substrate carrier to small lot size substrate carriers. For example, the substrates from one 25-substrate substrate carrier may be removed and placed in 25 one-substrate substrate carriers, 13 two-substrate substrate carriers, 9 three-substrate substrate carriers, 7 four-substrate substrate carriers, 5 five-substrate substrate carriers, 4 seven-substrate substrate carriers, etc. Each stocker 332-336 also may provide local storage for large and/or small lot size substrate carriers. Fewer or more stockers 332-336 may be employed. Additionally, the small lot size lithography bay 300 may be adapted to directly receive small lot size carriers from another small lot size bay of a fabrication facility (rather than from a large lot size transport system).

Operation of Small Lot Size Lithography Bay

FIG. 4 is a flowchart of an exemplary method 400 for operating the small lot size lithography bay 300. With reference to FIG. 4, in step 401, substrates are delivered to the small lot size lithography bay 300 via the large lot size transport system 338. For example, the large lot size transport system 338 may deliver substrate carriers that each hold 25 substrates to the stockers 332-336. The stockers 332-336 then may transfer substrates from large lot size carriers to small lot size carriers (e.g., from carriers that hold 25 substrates to carriers that hold 1, 2, 3, 4, 5, 6, etc., substrates).

Thereafter, in step 402, a first small lot size carrier is transported to one of the patterning tools 314-328 via the small lot size transport system 330. Additional small lot size carriers may be transferred to patterning tools 314-328 (sequentially or in parallel).

In step 403, the substrate or substrates contained in a first small lot size carrier are processed. Thereafter, in step 404, after the substrate or substrates of the first carrier have been processed at a patterning tool, the first small lot size carrier may be transported to one of the metrology and inspection tools 306-312 via the small lot size transport system 330.

In step 405, metrology and/or inspection of the substrate or substrates within the first small lot size carrier may be performed (e.g., to determine critical dimension or overly accuracy, defect level or type, etc.).

In step 406, it is determined whether an error condition is detected on a substrate, such as too many defects being present, improper patterning, etc. if an error condition is detected, in step 407, the first small lot size substrate carrier and its contents may be forwarded to the wet clean tool 302 and/or the dry strip tool 304 and re-worked (e.g., stripped, cleaned and prepared for processing within one or more of the patterning tools 314-328). If no error conditions are found on the substrate or substrates within the first small lot size carrier, in step 408, the first small lot size transport system 330 may return the carrier to the patterning tools 314-328 for more processing within the small lot size lithography bay 300 or to the stockers 332-336. At the stockers 332-336, substrates may be stored and/or transferred from small lot size carriers to large lot size carriers and transported to another bay of the overall fabrication facility (not shown) via the large lot size transport system 338. The method 400 then ends.

Data from the metrology and inspection tools 306-312 may be fedback to the patterning tools 314-328 to improve process performance (as shown in phantom by step 409 in FIG. 4). For example, if the critical dimension (CD) of a feature (e.g., of a resist layer) is too small or too large, then exposure time and/or resist processing conditions can be changed to improve process results. Other process parameters may be similarly manipulated. By using small lot sizes, feedback data can be provided to the patterning tools with less time delay than when large lot sizes are used. Faster feedback data can improve processing results more quickly and reduce scrap caused by mis-processing. This benefit is of particular importance for the most difficult and/or most critical lithography layers. For these layers, adjustment of process parameters based on feedback data from processed substrates may be necessary to meet process specifications.

The small lot size lithography bay 300 may include a controller 340 (FIG. 3) adapted to control operation of the lithography bay 300 as described above (e.g., by interfacing and/or controlling one or more of the re-work tools 302-304, the metrology and inspection tools 306-312, the patterning tools 314-328, the transport system 330 and/or the stockers 332-338). For instance, the controller 340 may be adapted to at least initiate and/or perform one or more of the steps of method 400 described previously. The controller 340 may be, for example, one or more microprocessors and/or microcontrollers, dedicated hardware, a combination of the same, etc., and may include suitable computer program code for controlling operation of the lithography bay 300.

The controller 340 may form part of an automation communications and/or software system that is provided for controlling operation of the small lot size lithography bay 300, and may include, for example, a manufacturing execution system (MES), a Material Control System (MCS), a Scheduler or the like. A separate delivery system controller (not shown) for controlling delivery of small lot size substrate carriers to the processing tools 302-328 may be employed. It will be appreciated that the delivery system controller may be part of the automation communications and/or software system described above; and/or that a separate MES, MCS or Scheduler may be employed.

Through use of small lot size carriers within the lithography bay 300, substrates may be processed within the patterning tools 314-328 and forwarded to the metrology and inspection tools 306-312 much faster than in a conventional, large lot size lithography bay. Substrates may be sent to metrology and/or inspection with little or no (in the case of single substrate carriers) delay (e.g., due to other substrates in a carrier being processed). In one embodiment, for example, metrology delays may be reduced from 200 plus substrates to less than 20 substrates. Further, through use of small lot size carriers, substrates may be transferred to different metrology tools in parallel. For example, when single substrate carriers are used, a first substrate may be forwarded to a critical dimension metrology tool, a second substrate may be forwarded to an overlay metrology tool, a third substrate may be forwarded to a defect metrology tool, etc., in parallel. Similar parallel operations may be performed with small lot size carriers that contain more than one substrate, without the large wait-time delays associated with large lot size carriers.

As another example, if a new mask setup is to be employed, the setup should be verified before processing many substrates with the mask setup (e.g., to avoid having to reprocess too many substrates if the setup is flawed). To perform setup verification, many substrates may be processed and sent to metrology. As stated, using small lot sizes significantly reduces the time required to receive metrology and/or inspection results for substrates. Accordingly, mask setup verification time is reduced. As an alternative approach, a new mask setup may be assumed to be valid, and substrates may be processed using the setup. Any errors in the setup will be detected following metrology on the processed substrates. Again, through use of small lot sizes, information from metrology and/or inspection is provided more rapidly so that the number of flawed substrates that must be re-worked (if any) is reduced. Quick, parallel feedback of information from metrology and/or inspection tools that perform different measurements (e.g., critical dimension, overlay, defect) may provide rapid, multiple and/or near simultaneous indicators of process conditions (e.g., to confirm setup, to monitor production processing, to detect process deviations, etc.).

The use of the small lot size lithography bay 300, and its associated reduced cycle time, also allows lot and substrate interlacing. For example, monitors and/or small lots may be interlaced without breaking the pipeline (e.g., the continuous sequencing of substrates within a process tool). If the pipeline of substrates is interrupted, such as by a space or gap in the continuous sequencing of substrates in the tool, then the tool may lose efficiency and net effective utilization may decrease. Further, monitoring may be increased and sampling improved without utilization loss. Single (or small lot size) substrate re-work may be included in the lithography bay 300.

In at least one embodiment of the invention, after substrates are exposed and/or patterned within a patterning tool 314-328, the substrates are moved to metrology as quickly as possible (e.g., without waiting for significantly more substrates to be processed as might be the case if a substrate carrier that holds 25 substrates were employed for transporting substrates within the lithography bay). Also, small lot size substrate carriers (e.g., single substrate carriers) may be sent to different metrology tools in parallel.

Note that stand alone metrology may be employed. Also, in some embodiments, dry strip and wet clean tools may be included in the line (as shown) to provide fast cycle time re-work when problems are discovered. Other small lot size substrate carrier delivery systems, apparatus and/or methods than those described may be employed.

Accordingly, while the present invention has been disclosed in connection with exemplary embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims. 

1. A method comprising: storing substrates in a plurality of small lot size substrate carriers, each adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates; and transporting at least one of the small lot size substrate carriers adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates within a lithography bay of a semiconductor device fabrication facility; wherein storing substrates in a plurality of small lot size substrate carriers each adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates comprises transferring substrates from a large lot size substrate carrier adapted to hold 13 or 25 substrates to a plurality of small lot size substrate carriers each adapted to hold less than a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates.
 2. The method of claim 1 wherein storing substrates in a plurality of small lot size substrate carriers, each adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates, comprises: (a) receiving a large lot size substrate carrier from a large lot size transport system, the large lot size substrate carrier adapted to store 13 or 25 substrates; and (b) transferring substrates from the large lot size substrate carrier adapted to store 13 or 25 substrates to a plurality of small lot size substrate carriers each adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates.
 3. The method of claim 2 further comprising performing steps (a) and (b) at a stocker.
 4. The method of claim 3 further comprising storing the plurality of small lot size substrate carriers at the stocker.
 5. The method of claim 1 wherein the plurality of small lot size substrate carriers are each adapted to hold a maximum capacity 4 or less substrates.
 6. The method of claim 1 wherein the plurality of small lot size substrate carriers are each adapted to hold a maximum capacity 3 or less substrates.
 7. The method of claim 1 wherein the plurality of small lot size substrate carriers are each adapted to hold a maximum capacity 2 or less substrates.
 8. The method of claim 1 wherein the plurality of small lot size substrate carriers are each adapted to hold a maximum capacity 1 substrate.
 9. The method of claim 1 wherein transporting at least one of the small lot size substrate carriers within a lithography bay of a semiconductor device fabrication facility comprises transporting a small lot size substrate carrier to a patterning tool.
 10. The method of claim 9 further comprising patterning a substrate from the small lot size substrate carrier using the patterning tool.
 11. The method of claim 9 further comprising transferring the small lot size substrate carrier to a tool adapted to perform at least one of metrology and inspection.
 12. The method of claim 11 further comprising performing at least one of metrology and inspection on a substrate from the small lot size substrate carrier.
 13. The method of claim 12 further comprising transferring the small lot size substrate carrier to one or more re-work tools based on results of performing at least one of metrology and inspection.
 14. The method of claim 13 further comprising re-working a substrate from the small lot size substrate carrier.
 15. The method of claim 4 wherein re-working the substrate includes at least one of wet cleaning and dry stripping the substrate.
 16. The method of claim 1 further comprising transporting a plurality of small lot size substrate carriers to patterning tools in parallel.
 17. The method of claim 1 further comprising transporting a plurality of small lot size substrate carriers in parallel to different metrology tools.
 18. The method of claim 1 wherein transporting at least one of the small lot size substrate carriers within a lithography bay of a semiconductor device fabrication facility comprises employing a continuously moving conveyor to transport the at least one small lot size substrate carrier within the lithography bay.
 19. A lithography bay comprising: a plurality of lithography tools; a small lot size transport system adapted to transport small lot size substrate carriers between the lithography tools, wherein each small lot size substrate carrier is adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates; and a stocker adapted to: (a) receive a large lot size substrate carrier from a large lot size transport system, the large lot size substrate carrier adapted to store 13 or 25 substrates; and (b) transfer substrates from the large lot size substrate carrier adapted to store 13 or 25 substrates to a plurality of small lot size substrate carriers each adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates.
 20. The lithography bay of claim 19 wherein the lithography tools comprise patterning tools and at least one of metrology and inspection tools.
 21. The lithography bay of claim 19 wherein the lithography tools comprise at least one re-work tool.
 22. The lithography bay of claim 21 wherein the at least one re-work tool is in-line with other lithography tools.
 23. The lithography bay of claim 21 wherein the at least one re-work tool includes a wet cleaning tool and a dry stripping tool.
 24. The lithography bay of claim 19 wherein the stocker is adapted to store at least one of large lot size carrier adapted to store 13 or 25 substrates and at least one small lot size substrate carrier adapted to hold a maximum capacity of 1, 2, 3, 4, 5, 6, or 7 substrates.
 25. The lithography bay of claim 19 wherein the small lot size transport system is adapted to transport small lot size substrate carriers to the stocker.
 26. The lithography bay of claim 19 wherein the small lot size substrate carriers are each adapted to store a maximum capacity of 4 or less substrates.
 27. The lithography bay of claim 19 wherein the small lot size substrate carriers are each adapted to store a maximum capacity of 3 or less substrates.
 28. The lithography bay of claim 19 wherein the small lot size substrate carriers are each adapted to store a maximum capacity of 2 or less substrates.
 29. The lithography bay of claim 19 wherein the small lot size substrate carriers are each adapted to store a maximum capacity of 1 substrate.
 30. A method comprising: storing substrates in a plurality of small lot size substrate carriers, each adapted to hold less than a maximum of 13 substrates; and transporting at least one of the small lot size substrate carriers within a lithography bay of a semiconductor device fabrication facility; and maintaining a predetermined work in progress level within the semiconductor device fabrication facility by: decreasing an average cycle time of high priority substrates within the lithography bay so as to approximately maintain the predetermined work in progress level; wherein storing substrates in a plurality of small lot size substrate carriers comprises transferring substrates from a substrate carrier adapted to hold up to 25 substrates to a plurality of small lot size substrate carriers. 